FIG. 28 shows a simplified structure of a printed wiring substrate 100 of a mobile wireless terminal including a wireless circuit and a digital circuit.
The printed wiring substrate 100 includes a signal wire/power layer 101 and a ground layer G in order to achieve the functionality of a mobile wireless terminal. A wireless circuit 102 and a digital circuit 103 are mounted on the upper surface of the ground layer G. Since the ground layer G is shared by the wireless circuit 102 and the digital circuit 103, a signal transmitted from the digital circuit 103 may include a large number of high-frequency components, causing an undesired high-frequency current (i.e. a noise current) which may be transmitted through the ground layer G and enter into the wireless circuit 102. An antenna and a power circuit (not shown) are mounted on the printed wiring substrate 100.
A mobile wireless terminal utilizes a balanced antenna such as a dipole antenna disclosed in Patent Literature 1. In a noise suppression structure shown in FIG. 29, a balanced antenna 110 is connected to the wireless circuit 102 via a coaxial line 111, wherein the coaxial line 111 is configured of an outer conductor 111A and an inner conductor 111B. A sleeve 113 of a cylindrical shape whose terminal end is short-circuited with a doughnut-type short-circuiting plane 112 is used as the outer conductor 111A of the coaxial line 111. The leading end of the sleeve 113 is an open end 113A.
To suppress an undesired high-frequency current i flowing through the outer conductor 111A of the coaxial line 111, the sleeve 113 may function as a balun connected between the balanced antenna 110 and the coaxial line 111 of an unbalanced line. The length l of the sleeve 113 is equal to a quarter of the wavelength λ of an undesired high-frequency current, wherein it is possible to prevent the flowing of an undesired high-frequency current i because of an infinite input impedance of a sleeve open end at an operating frequency of the balanced antenna 110 in theory. This structure suppresses electromagnetic noise occurring due to an inconsistency of balance-unbalance conversion.
Patent Literatures 2 to 4 are listed as technologies for suppressing the foregoing undesired high-frequency currents (i.e. noise currents).
A mobile wireless terminal disclosed in Patent Literature 2 is equipped with an antenna substrate, an antenna element serving as an antenna pattern formed on the antenna substrate, a coaxial cable feeding power to the antenna element, and a dielectric sleeve attached to the distal end of the coaxial cable, all of which are attached to the surface opposite to the surface of a printed wiring substrate mounting a speaker thereon.
A mobile wireless terminal disclosed in Patent Literature 3 is designed such that a coaxial cable is connected to an antenna/printed wiring substrate and covered with a bazooka. The bazooka is designed such that a dielectric inner cylinder is engaged with a conductor inner cylinder in a dielectric outer cylinder, wherein the lower end of the dielectric outer cylinder and the conductor inner cylinder is a short-circuited surface while the upper end thereof is an open end. Herein, the length of the dielectric outer cylinder and the dielectric inner cylinder is reduced from “¼λ” to “1/√∈r” owing to a dielectric constant ∈r.
A mobile wireless terminal disclosed in Patent Literature 4 includes a coaxial cable feeding power to first and second antenna elements formed on an antenna substrate, wherein first and second resonating conductors are separated from each other and disposed in the longitudinal direction of the coaxial cable, thus forming a dielectric sleeve connected to the coaxial cable in which the distal end is open while the other end is short-circuited. The length of the dielectric sleeve is reduced from “¼λ” to “1/√∈r” owing to a dielectric constant ∈r.